There are prior art photovoltaic concentration systems wherein the solar radiation is concentrated using an optical system on the photovoltaic material. The solar concentration factor of the optical concentration systems, defined as the ratio of the area of the inlet opening of the solar radiation and the area occupied by the photovoltaic material on which the radiation is concentrated, may range between values of approximately 1 and 10, which are referred to as low concentration, up to values of 100 and above (high concentration), passing through intermediate values of between approximately 10 and 100 (average concentration).
Patent document AU-A-70929/87 and the article “Prism-coupled compound parabola: a new ideal and optimal solar concentrator”, Ian R. Edmonds, OPTICS LETTERS, Vol. 11, No. 8, August 1986, the contents of which are referred to fully in this invention, describe a 2-dimensional (2-D) solar concentrator for low concentration photovoltaics comprising a pair of parabolic reflectors which mirror each other, each parabola, in a cross section at right angles, having an axis parallel to the direction of incidence of the radiation incident at the predetermined angle of acceptance and focus on the apex of a refractive prism positioned between the two reflectors and having base on the optical outlet of the concentrator (at which the photovoltaic cells are positioned). This structure, according of the author of the above-mentioned documents, allows the collection on the photovoltaic cells of all the radiation incident within the angle of acceptance.
The Applicant has found that the prior art solar concentrators, including the above-mentioned concentrator, are not without disadvantages and can be improved from various points of view.
In general, the Applicant has found that prior art solar concentrators are characterised by an unsatisfactory concentration factor and/or concentration efficiency and/or by a large overall size, in particular the height along the axis of symmetry (see below), and/or by a complex production and/or installation and/or maintenance and/or by a high production cost.
In particular, the Applicant has found that the structure of the above-mentioned concentrator described by Edmonds is not in reality optimal in terms of ‘concentration efficiency’ (Pr), defined as the ratio between the total optical power incident on the optical inlet of the concentrator and the optical power which actually reaches the optical outlet of the concentrator (and, therefore, the photovoltaic cell).
Firstly, the Applicant notes that the above-mentioned prior art 2-dimensional concentrator has a plane of symmetry, a main extension along a longitudinal axis belong to the plane of symmetry and a continuum of cross-sections at right angles to the longitudinal axis between them equal along the entire axis of longitudinal extension, the cross-section of the plane of symmetry defining an axis of symmetry of the right-angled cross-section. In this context, the radiation incident on the concentrator is characterised by two angles of incidence with respect to the axis of symmetry of the cross-section of the concentrator, measured on two planes at right angles to each other, where the first angle (indicated hereafter as θNS), considered also by Edmonds, is measured on the plane of the cross-section at right angles (on which the above-mentioned parabolas are defined, see, for example, FIG. 1 of the document AU-A-70929/87) and the second angle (indicated as θEO) is measured on the plane of symmetry of the 2-D concentrator. More precisely, the actual angle of incidence with respect to the axis of symmetry can be projected on the two above-mentioned planes at right angles. For simplicity, the projections will be referred to in this invention with the expression “first and second angle of incidence”.
According to Edmonds, the concentration efficiency of the above-mentioned concentrator is equal to one for first angles of incidence θNS less than the angle of acceptance. The Applicant found that this statement is strictly true (neglecting the losses due to reflection and refraction) only when the radiation has a direction of incidence which lies on the cross-section plane of the concentrator, or, in other words, the second angle of incidence θEO, formed with this cross-section plane, is zero.
When, however, the radiation forms a second angle θEO with the cross-section plane different from zero, the Applicant has surprisingly discovered that the behaviour of the concentrator differs from that considered by Edmonds (for radiation incident with a first angle θNS, on the cross-section plane, different from zero), with a reduction in the concentration efficiency which reduces further the greater is the above-mentioned second angle of incidence θEO (and the greater is the above-mentioned first angle θNS).
The Applicant has realised that in a real application of the above-mentioned prior art 2-D concentrator, wherein the latter is oriented with the longitudinal axis of extension (along which the cross-section geometry remains constant) parallel to the East-West geographical direction and with the above-mentioned cross-section at right angles parallel to the North-South geographical direction, the solar radiation over the span of one day varies the relative second angle of incidence (θEO) up to a maximum ranging from −90° to +90° (with clear horizon). Therefore, according to the Applicant, the overall efficiency over the span of the entire day of the above-mentioned prior art 2-D concentrator would not be as expected, except in the ideal case in which the first angle of incidence θNS is zero (concentrator orientated in such a way that the elevation of the axis of symmetry is equal to the elevation of the sun, a condition which may occur at the most only for a few days a year). On the other hand, the overall efficiency would be typically less than that expected.
A problem which forms the basis of this invention, according to its various aspects and/or embodiments, is to provide a solar concentrator for photovoltaics, a photovoltaic system, a photovoltaic panel and a method for producing a photovoltaic panel that can overcome one or more of the above-mentioned disadvantages.